If you spend some time on the internet before the trip, you can find many discounts and good deals online, and as a result save a decent amount on your trip. For example, if you buy admission tickets to NYC sights online in advance, you can save up to 50%, savings for Broadway shows can be up to 30%, and NYC sightseeing tours offer discounts up to 15-20% less than their regular price. If you decide to purchase online, it is better do it before you go so that you could print out all confirmations and e-tickets, as it might not be possible at the hotel.
There are almost always some kind of sales in New York City stores, but if you are looking for a mega sales event, come to New York several days before the major holidays: the Fourth of July, Labor Day (the first weekend in September), St. Valentine’s on February 14th, Easter or Memorial Day (last weekend in May). The largest sales event of the year is held between Thanksgiving Day (fourth Thursday in November) and New Year’s. It is considered that the biggest discounts are on Black Friday (the day right after Thanksgiving).
Tickets to Broadway, the Metropolitan Opera and Carnegie Hall are definitely worth buying online, the earlier before the event, the better. The closer to the performance, the higher the price and the fewer tickets and good seats left. There is a place at Times Square where they sell discount tickets to Broadway shows, TKTS: the price is really good, but will you might lose up to two hours in the line.